include_docs should default to True in Foo.view

This was the behavior before this commit (between 0.6.1 and 0.6.2): https://github.com/benoitc/couchdbkit/commit/b88a6c4da90667cc5f0d8c7df9bcdc826b66dabd

Also, this is strictly better than the current behavior when you don't explicitly pass in `include_docs=True` which is:

```
Traceback (most recent call last):
  File "/Users/droberts/couchdbkit/tests/client_test.py", line 869, in test_document_view
    Foo.view('_all_docs').all()
  File "couchdbkit/client.py", line 984, in all
    return list(self.iterator())
  File "couchdbkit/client.py", line 947, in iterator
    yield wrapper(row)
  File "couchdbkit/client.py", line 928, in row_wrapper
    return schema(data)
  File "couchdbkit/schema/util.py", line 29, in wrap
    cls = classes[doc_type]
KeyError: None
```